The objectives of the animal experiments are: 1) to study the pathophysiologic basis and consequences of venous changes in experimental hypertension, and 2) to investigate the role of neural stimuli and of circulating humoral substances in the pathogenesis of vascular (venous) changes. The composition of veins and the effect of reversal of hypertension on venous composition will be studied in hypertensive rats. An attempt will be made to prevent the development of venous changes by the selective destruction of the peripheral sympathetic ganglia with guanethidine in neonatal rats. In parabiotic rats, one hypertensive and one normotensive control, venous changes will be sought in the control rat. Rabbit aortic media and dog saphenous vein explants will be cultured in artificial media containing 10 percent dog serum obtained from the same dog while the dog was normotensive and after the induction of renovascular hypertension. The effect of the sera on the histology and the chemical composition of the explants will be examined. In the dog, the immediate effect of unilateral renal artery constriction on venous smooth muscle contractile state will be studied by perfusing the saphenous veins in vitro with venous blood. Venous pressure-volume curves will be inscribed before and after renal artery constriction. In chronically hypertensive rats and in dogs immediately after renal artery constriction, the distribution of intravascular volumes (cardiopulmonary and peripheral) will be measured. The objectives of the human experiments are: 1) to confirm reports of decreased venous distensibility, using venous plethysmography, in essential hypertension, 2) to correlate venous distensibility measurements with systemic hemodynamic data in hypertensive subjects, and 3) to analyze the water, electrolyte and connective tissue composition of veins in human hypertension.